Checking if ELF file is executable is never done

[fedi-nabli]

In lecture 94, Implementing the ELF Loader - Part 2, we create the funciton static bool elf_is_executable(struct elf_header* header) since we are only working with executable files, but later at the video, when creating the int elf_validate_loaded(struct elf_header* header) function, we do not call it to validate the file format.